Mobile radio communications systems generally comprise a number of mobile or portable radio units. The radio units in a public- or private mobile radio communications system are often referred to as ‘mobile stations’. The radio communication system's infrastructure comprises fixed nodes, termed ‘base stations’, through which mobile stations can communicate.
Normally, a mobile station is within communication range of a base station. In this case, the mobile station will communicate through the base station, this mode of operation typically being termed ‘trunked mode’. However, some mobile radio communication systems allow an individual mobile station to set up a direct radio link to another mobile station, without the communication link passing through the infrastructure of the communication system. This form of communication between two mobile stations is referred to as ‘direct mode’ operation.
Mobile stations operating in direct mode have to receive radio signals that typically show greater variation in their parameters than radio signals received from the infrastructure of the communications system. This is because of differences in the signals transmitted from a mobile station in comparison to those from a base station of the infrastructure. As an example, the lower quality clock of a mobile station may cause greater variation in the carrier frequency of the signal broadcast by the mobile station in comparison to that from a base station.
The information carried by any digital communications system can only be recovered after the receiver has first synchronised to the received signal. The high frequency signal reaching the receiver will be at a frequency that depends on the frequency of an oscillator in the transmitter. A further oscillator in the receiver is used to down-convert the high frequency received signal. Because the oscillators in the transmitter and the receiver do not work at precisely the same frequency, the down-converted signal in the receiver has a slightly different frequency than that which would arise if the two oscillators had identical frequencies. This mismatch is part of the reason why a synchronisation circuit is needed in the receiver, to ‘compensate’ for the down-converted signal not having its ideal, theoretical frequency value. The mismatch will typically vary with time.
Thus there is a need in a mobile digital radio system for synchronisation in the radio receiver. The state where synchronisation between the receiver and the received signal has been attained is often referred to simply as ‘lock’. The need for accurate synchronisation is particularly great for direct mode operation of mobile stations, because of the greater variation in the parameters of the received radio signals. This applies both to:    (i) achieving synchronisation when a call is first set up; and    (ii) maintaining synchronisation during a call.
In the Terrestrial Trunked radio system (TETRA), direct mode operation is permitted. A direct mode call is set up by sending two frames (8 slots) of synchronisation bursts, which last for 113 msec. In terms of the requirements on the automatic frequency correction (AFC) algorithm in the receiving radio, the radio needs to synchronise to these bursts, in order to start decoding data as quickly as possible. After synchronisation at call set up, the receiver must then maintain synchronisation for the duration of the call. This requires the receiver to follow variations in the frequency of the received signal. This action of following is often referred to as ‘tracking’. The part of the receiver which carries this out is the ‘tracking loop’.
The synchronisation bursts are an example of a ‘training sequence’. A ‘training sequence’ is a sequence of symbols in a communications signal which are known to both the transmitter and the receiver.
FIG. 1 shows a prior art arrangement of a frequency tracking loop 10. This is a ‘feedback’ tracking loop. FIG. 1 shows a generalised arrangement in order to explain the principle of operation of a frequency tracking loop.
Briefly, the elements shown in FIG. 1 are:    (i) A mixer 22. One input to the mixer, element 20, provides the radio signal received by the receiver, which may have been ‘down-converted’ to intermediate frequency. This down-converted signal is the ‘input’ to the frequency tracking loop. The oscillator used to down-convert the received high frequency signal to intermediate frequency is not shown in FIG. 1. It is however this oscillator which is partly responsible for frequency variations in the input signal. The output of the mixer is provided on output 24.    (ii) A ‘data matched’ filter 26. The filter is matched to the pulse shape of the received signal. This means that it is designed to filter pulses of the shape transmitted by the transmitter of the radio system.    (iii) A decimation circuit 30. The decimation circuit 30 reduces the number of samples supplied to it from the filter 26. Decimation circuit 30 reduces the number of samples to one per received pulse. This is enough to identify each received pulse.    (iv) A Viterbi & Viterbi Frequency estimator 34. This is a known circuit element, which implements a known method of measuring frequency offset. Estimator 34 uses the training sequences in making the estimate of frequency offset. Other prior art arrangements are however known which do not rely on the training sequences to achieve this.    (v) A complex phase determining and scaling circuit 38.    (vi) A loop filter 42.    (vii) An oscillator 46. The output of oscillator 46 is fed back on output 48 to provide the second input to mixer 22.
The purpose of the circuit shown in FIG. 1 is to ‘track’ variations in the frequency of the input signal. Elements (ii)–(vi) drive oscillator 46 to generate a signal which matches the frequency of the input signal. As there is a time delay in the oscillator generating a signal that has the same frequency as the input to the frequency tracking loop, the ‘following’ action provided by the whole frequency tracking loop is not instantaneous.
In operation of the feedback tracking loop, the frequency estimator 34 and phase and scaling block 38 give an estimate of the frequency offset from the current input. This estimate will be noisy.
The noisy estimates are passed through the loop filter 42, which reduces the noise and defines the transient response of the frequency tracking loop 10.
The bandwidth chosen for the loop filter 42 is a compromise between two requirements. One of these is the requirement to reject noise. This dictates that the loop filter 42 have a narrow bandwidth. The other requirement is that the frequency tracking loop 10 be able to track a changing input frequency. This dictates that the loop filter 42 have a wide bandwidth. Clearly these requirements set conflicting conditions on the loop filter 42.
Frequency tracking loops of the prior art are known, which switch the bandwidth of the tracking filter during acquisition of a signal. A wide bandwidth is used to acquire the signal, and the bandwidth of the loop filter is narrowed during tracking.
Frequency tracking loops are also known in the prior art which adapt the loop filter bandwidth in dependence on the size of the estimate of frequency. However, adaptation of the loop is based on the measurement of the error between the currently received and currently generated frequencies. This measurement itself may be in error and can lead to an adaptation that suppresses the wanted signal.
A need exists to alleviate problems of the arrangements known from the prior art. One particular problem addressed by the present invention is to provide optimum frequency tracking in a feedback tracking loop.